Adding more subtitles doesn't work in vanilla.
Subtitles for mods that add sounds won't work.

• • What I expected to happen was...: **

Making two separate sound events have two different subtitles, such as the preexisting subtitles.block.door.toggle being replaced instead with subtitles.block.door.wood.open and subtitles.block.door.wood.close

• • What actually happened was...: **

All subtitles that have the exact same name as before can be edited, but ones that are replaced/renamed in sounds.json are ignored, even if the replace: true tag is present. They should be replaceable

• • Steps to Reproduce: **

(To clarify, existing subtitles can be edited. But new subtitles don't get recognised.)

1. Rename any subtitle file in sounds.json (subtitles.block.door.toggle to subtitles.block.door.somethingelse)
1.5 Optional - Adding replace: true, tag to the door sound event.
2. Edit subtitles.block.door.toggle to subtitles.block.door.somethingelse in relevant language file that contains the subtitles.
3. Change the subtitle to something else. Door Does Something Else for example.
4. Ingame, observe the subtitle remaining unchanged.

ALSO :

Completely new subtitles (such as attack sound subtitles) fail to show up, even if they are introduced in sounds.json